1. Field of the Invention
This invention relates to an apparatus for electro-statically applying an anti-sticking agent on annealed oriented electrical sheet steel in coil.
2. Description of the Prior Art
Oriented electrical sheet steel made from hot-rolled strip by applying one, two or more cold reduction and annealing processes is further subjected, in coil form, to annealing at elevated temperatures to develop the crystals in (110) and [001] orientations. The strip to undergo this elevated-temperature annealing is coiled after an anti-sticking agent is applied on the exit side of the continuous annealing furnace to prevent the occurrence of undesirable seizure between individual wraps of the coil.
Magnesia, alumina and some other refractories have been used as the anti-sticking agent. Aqueous suspensions of such refractories are applied and dried on the surface of the strip. Especially magnesia has been in popular use as it forms a glassy film consisting essentially of forsterite (2MgO.SiO.sub.2) reacting at high temperatures with a layer of scale, consisting principally of silica, formed on the surface of the strip.
When magnesia is suspended in water, part of it is hydrated into magnesium hydroxide, which, in the subsequent annealing process, gets decomposed and excessively oxidizes the surface of the strip, thereby damaging the magnetic properties of the steel and the uniformity of the glassy film.
To overcome such disadvantages, the methods disclosed in the Japanese Patent Public Disclosure Nos. 12211 of 1964 and 11393 of 1982 electrostatically apply powders of anti-sticking agent on the surface of steel strip to be annealed. The method of the Japanese Patent Public Disclosure No. 12211 of 1964 applied a powder of anti-sticking agent on the surface of steel strip by introducing the powder into a space between the strip and an electrode discharging a positive corona. The method of the Japanese Patent Public Disclosure No. 11393 of 1982, on the other hand, applies a small quantity of slurry consisting principally of magnesium oxide on the surface of strip in order to assure the forming of a good glassy film. Then, an electrically charged powder of anti-sticking agent is applied on the dried undercoat on the oppositely charged strip of oriented electrical steel.
Another method disclosed in the Japanese Provisional Patent Publication No. 128213 of 1985 electrostatically applies a charged powder of anti-sticking agent on the surface of oppositely charged strip of oriented electrical steel using a spray gun. This method applies the charged powder to the under side of the strip at a point closer to the coiler than the roll on the entry side thereof. Considering space limitations and other factors, the gun 4 shown in FIG. 1 of the same publication is installed in the optimum position. But the specification states that the object of the invention can be achieved even if the gun is not placed in that specific position but in other position between points A and B. The specification also says that the powder spray gun and dust-collector (not shown) should preferably be movable with a change (i.e., an increase) in the diameter of a coil.
However, the Japanese Provisional Patent Publication No. 128213 of 1985 shows neither any concrete mechanism by which the spray gun and dust-collector follow up a change in the coil diameter nor the best type of such dust-collector. An improperly designed dust-collector may draw out the powder of anti-sticking agent that is flying toward the strip, thus materially lowering the rate of powder adhesion of the strip.